1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method using the same.
2. Description of the Related Art
Along with the recent increase in the packing density of semiconductor devices, an exposure apparatus used in lithography for manufacturing these devices has come to use a pulsed laser such as a KrF or ArF excimer laser as a light source having a wavelength in the far-ultraviolet region, in place of a mercury lamp. In an exposure apparatus which uses an excimer laser, the excimer laser light source and the exposure apparatus major portion can be connected via an interface cable such as an optical fiber. The exposure operation of such an exposure apparatus can be controlled by a control device provided to the exposure apparatus major portion. The exposure apparatus major portion illuminates a reticle with pulsed laser light emitted by the excimer laser. The pattern of the reticle illuminated is projected onto a wafer coated with a photosensitive agent by a projection optical system, thereby forming a latent image on the resist.
The laser chamber of an excimer laser is generally filled with a mixed gas containing three types of gases: a halogen gas such as fluorine gas, an inert gas such as krypton or argon gas, and a rare gas such as helium or neon gas. The excimer laser light source emits pulsed laser light having a pulse width on the order of nanoseconds by performing electric discharge in the chamber so that the halogen gas and the inert gas react with each other.
The photon energy of excimer laser light is 114.1 kcal/mol for KrF (248 nm), 147.2 kcal/mol for ArF (193 nm), or 180.1 kcal/mol for F2 (157 nm). In contrast, the binding/dissociation energy of molecules is, for example, 84 kcal/mol for the carbon-carbon bond. In other words, the optical energies in this wavelength range correspond to the bonding/dissociation energies of various substances. For this reason, irradiating various substances with excimer laser light readily cause light absorption or photochemical reactions of these substances. Because of this property, light having a wavelength in this wavelength range is often used in processing (e.g., CVD and etching) of various substances. Also, because the optical characteristics such as the absorption and reflection characteristics differ among substances, excimer laser light can also be used in structure analysis of the substances.
However, exposure light having a wavelength in the wavelength range of excimer laser light is absorbed by oxygen or moisture in the atmosphere. This is because the photon energy of the exposure light increases as the wavelength of the exposure light shortens. An increase in the photon energy activates photochemical reactions which involve the substances present in the optical path. This makes it necessary to purge the interior of an optical unit, through which the exposure light passes, using an inert gas such as nitrogen or helium gas (Japanese Patent Laid-Open No. 2001-284235). Although light absorption and the like can be suppressed by the purge, it is difficult to perfectly eliminate all impurities present in the optical unit. The optical unit is contaminated upon, for example, the light absorption of the substances adhering on the optical unit or the decomposition or generation of substances due to photochemical reactions. In addition, if a nitrogen gas atmosphere contains water vapor, irradiating the optical unit with ultraviolet light generates ammonia. The optical unit irradiated with laser light is contaminated or deteriorated due to these factors.
Conventionally, because the exposure apparatus is not imparted with a function of detecting deterioration in the optical performance in advance, it is determined that an optical unit of the exposure apparatus has come to the end of its lifetime by confirming that an abnormality has occurred in an exposure process or measurement process. It is therefore necessary to specify and prepare components necessary for restoration after an abnormality has been detected and to restore the normal operation of the apparatus. It takes a long time to reactivate the exposure apparatus. Also, when an exposure process is performed using an optical unit which has already come to the end of its lifetime, defective products may be manufactured due to deterioration in the optical performance.